my fiancé wants a tattoo covering most of his back and going onto his chest of a Chinese dragon his mum used to do tattoos but had to stop after being poorly and it's hard for him to get one covered up atm because for some reason no tattooist will go over them,can some explain why this is?

Noo. It's from Dexter's laberatory. Remember? The episode with the ponies and bikers/action heroes or whatever. ^^
Me and my sister both decided when she was like, 8 and I was 6 that when we grew old enough so would we get that tattoo somewhere. And now we have! ^^